Okay, i thought i had a pretty good grasp on how thermistors work and behave, until today...

I've assembled my arcol.hu v4 hotend and went on to test the heater and thermistor.Setting the thermistor in Marlin to one of the 100k defines (thought it doesn't matter much which for a first test, just wanted to see if connections are okay and thermistor is working), i got a reading of 68-70 deg C at ambient temperature of 30 deg C.

Okay, i thought, maybe the 2printbeta/arcol.hu guys screwed something up and packed a 10k thermistor...So i reconfigure Marlin (instead of thinking it through - haven't had my coffee yet) and - lo and behold - i get a reading of 15 deg C.

Thinking maybe it's just faulty, i turn the heater on and set it to 100 deg C and temperature starts rising rather quickly, hitting the pain threshold at about 85-90 deg C. That's 45-50 deg C, which is more or less the expected value, meaning the reading is just linearly shifted up by about 40 deg C in that range. Or is it...

Okay, let's check the thermistor resistance... Here's where things start getting really weird for me.I hook up a multimeter, set it to 20k range and the readout starts steadily climbing, from 0 kOhm to 15 kOhm, at which point i disconnected it, set it to the 200k range and reconnected it. And it shows - i shit you not - MINUS 30k rising towards 0k. So i think, perhaps it's an artifact of different voltages the multimeter is applying to the test leads at different ranges, or something (i don't know, don't look at me like that, i still haven't had my coffee). I reconnect the multimeter and set it to volts and measure a steady -20mV on the thermistor. Now the weird bit. I swap the leads, get +20mV, switch to ohms and get a normal reading.

As if that isn't making my morning bizarre enough, the reading i get is around 67 kOhms. Indicating i'm quite probably stuck with a 50k thermistor under 2cm of fire cement that i currently have no will nor intention of getting off the hotend.

I'm guessing it's probably a Semitec 503GT-2.

Discounting all the bizareness with measuring the resistance (polarized thermistors? first time i've heard of that) and assuming it is a 50k one, do you guys think that will have enough resolution at ABS temps? Furthermore, is there a convenient pre-compiled table for 50k thermistors out there or should i just generate my own with createTemperatureLookup.py?

Last edited by orcinus on Tue Jul 17, 2012 6:53 pm, edited 1 time in total.

Hm. I did some quick number crunching and the resistance values i'm measuring don't fit a 50k thermistor either.Either it's a bad part, or there might be a tiiiny, slight, off chance that there's still some moisture stuck in the fire cement.

I did let it dry out for about 72 hours, but it might still need some "baking". I'll leave the hotend heated up to 200-ish deg C or so for a while.

Failing that, i can either measure/calculate the beta and Rz and compile a table for this particular thermistor (whatever 0-power @ 25 it might turn out to be) or tear the fire cement down, replace the thermistor and use high temp silicone this time around, because i don't have any fire cement left.

Last edited by orcinus on Mon Jun 25, 2012 2:44 am, edited 1 time in total.

Looks like it was fire cement after all. I started heating the hot end up while tracking the temperature with a thermocouple stuffed inside the head. The value from the thermistor and the value from the thermocouple started to converge towards around 110 deg C and were tracking each other within 5 degrees beyond that point.

At around 150 deg C, the lump of fire cement belched out some steam - it looks like there was a pocket of moisture somehow stuck inside. I left the hot end to simmer at 230 for a while and then left it to cool off.

At the moment of this writing, the thermistor reads 43 and the thermocouple 41-42, which is acceptable, i guess.I'm too sleepy to wait for it to cool down all the way (and/or go and get some ice), so further testing's gonna have to wait tomorrow.

But it seems the case of the wonky thermistor is pretty much solved and closed

Yup, the problem's completely gone after heating up the hot end up to 200 deg C one more time.

Interesting factoid - after wrapping the nozzle in PTFE tape, the temperature inside the nozzle overshoots the temperature of the heater block by 5 deg C and cools down slower than the block. Meaning it's having a pretty noticeable effect on the thermal conductivity between the block and the nozzle, despite being pretty thin.

I'm thinking of removing it. The Arcol.hu wiki suggests wrapping the nozzle in it before screwing it in to prevent aluminium bonding at high temperatures. From what i've read, the temperatures needed for thermocompression bonding are much much higher than anything that nozzle's ever gonna experience (400+ deg C), so i'm not completely convinced the PTFE tape is necessary.

For future reference, you can't measure resistance when the device under test is in a powered circuit, and that includes a circuit with a capacitor storing energy. That would perfectly match your symptom of reversing the multimeter leads to a thermistor and reading a negative resistance that's trending toward zero.

Oh, if only it were that easy to make negative resistances.

I'm glad the final thermal cure and belched steam corrected your problem and you are good to go.

I was just wondering a few minutes ago about using high temperature anti-seize compound where a brass nozzle threads onto a stainless barrel on an extruder. I have no practical experience with extruders yet, but stainless is a galling metal, and brass can corrode, particularly at high temperatures. Any time two dissimilar metals are in contact, there is the potential for galvanic corrosion, and adding heat probably isn't going to make that any better. If you don't want to use PTFE tape as an anti-seize element because it's acting as a thermal insulator, you might try some high temperature anti-seize compound. It's copper colored grease. The copper should give it good thermal conductivity. You can buy high temperature anti-seize compound at most auto supply stores.

This thread makes me glad I bought a MakerGear extruder (hopefully a known quantity) and I have four pre-assembled QU-BD extruders on order. I have enough to contend with climbing the 3D printing learning curve without a day spent in temporary thermistor insanity... although I ordered four thermistors today from Ultimachine to use with MK 1 heated build platforms, so I may yet experience thermistor insanity.

I was measuring it unpowered, of course (or i would've been reading voltages much higher than 20 mV)

But your comment got me thinking. I wonder if the wet fire cement between the thermistor leads was acting as a capacitor. It would make perfect sense - electrolyte between two electrodes.

Thanks for the anti-siezing tip. I was thinking of maybe greasing the nozzle threads with thermal paste. That would conduct heat nicely, while keeping at least some separation between the threads. I gave up on it in the end because most thermal pastes degrade pretty fast above 100 deg C.

Oh, BTW, i've had absolutely no issues with the heated bed thermistor. The HBP worked perfectly the first time i've hooked it up. Although i was out of 1k 0805 resistors, so i had to squeeze in 2 2k ones in parallel on the solder pads meant for one, which was a bit of a drag.

orcinus wrote:I wonder if the wet fire cement between the thermistor leads was acting as a capacitor. It would make perfect sense - electrolyte between two electrodes.

That would be more of a battery, although not a very good one. Low voltage, and it apparently couldn't source enough current to overpower an ohmmeter.

My LAVA heated build platform arrived today, so maybe I wont need to mess with thermistors after all, but from past experience on the bleeding edge, I fully expect to be twiddling with everything... including thermistor wiring and calibration.

The electrodes would have to be made of different materials (i.e. have different electronegativity) for it to be a battery.Then again, i don't have a clue how two Cu electrodes with electrolyte between them would make up a polarized capacitor.

Weirdness

I'm still waiting for my Lava. Not in too much of a hurry, waiting for stuff to ship from Canada to Croatia usually takes time, so i'm used to it.Plus, i already have a Heatbed MkII, so it's not like i desperately need it to complete my Hadron.

Poor thing's completed, built and mostly calibrated (i still need to tweak out the wobble out of the Z axis - i've modified the couplers with some cable sleeving so i can tweak the angle a little and have it absorb the wobble), but is sitting in the corner waiting for my current work-week-chaos to end.